The present invention relates to cavity resonators and particularly their use in oscillator circuits.
Resonators are important components, which may be used in greatly varying applications. Thus, for example, microwave systems require high-quality resonators, which are used in filters and oscillator circuits. A choice must be made between cavity resonators and dielectric resonators, the size, the weight, the costs, and other aspects being able to play a role.
Cavity resonators in the various known embodiments are subject to a change of the resonance frequency in the event of temperature change, which is undesirable for most applications. A temperature change may result due to a change of the ambient temperature, due to a temperature change in an integrated oscillator circuit, or due to losses which occur in the resonant cavity. A change of the dimensions of the resonator results due to a temperature change, which results in the cited change of the resonance frequency.
There are various approaches for reducing the temperature influence on resonators. For example, it is possible to reduce the resonance frequency change caused by a temperature change by inserting a dielectric part into the cavity, the dielectric part having to have a suitable temperature coefficient of the dielectric permittivity.
Another possibility is to construct a cavity from various materials having different temperature expansion coefficients. This possibility is well-known and is used for “coaxial reentrant cavity” resonators. An example of such a resonator is described, for example, in Japanese Patent JP 52075154, which was published on Jun. 23, 1977. FIGS. 1A and 1B show a resonator 10 according to this Japanese patent in a greatly simplified illustration. As may be seen from FIGS. 1A and 1B, there is a rod 12, which penetrates coaxially into a cavity 11 of the resonator 10. A state having lower temperature T is shown in FIG. 1A. If the temperature is increased to T′, the cavity 11 expands, as indicated by arrows around the circumference in FIG. 1B. The rod 12 becomes longer in the event of a temperature increase. If the materials of the cavity 11 in the rod 12 are selected so that the rod 12 experiences a smaller expansion, the capacitive gap (area 13) between the lower rod end and the lower wall of the cavity 11 becomes larger. This change of the capacitive gap (reduction of the capacitive load of the resonator in the event of a temperature increase) in the area 13 results in the resonator frequency of the resonator 10 remaining relatively constant in a specific temperature range. A disadvantage of such a reentrant cavity resonator 10 is the relatively poor quality factor Q. Especially at high frequencies above 10 GHz, the quality factor Q worsens noticeably because of the high field concentration in the capacitive gap and its immediate surroundings.
There are other resonators which are equipped with means for compensating for the temperature influence. These types of resonators are also referred to as “clamped cavity” resonators. An example of such a resonator may be inferred from U.S. Pat. No. 2,528,387. The cavity of the resonator is designed according to this approach in such a way that the geometric changes which would normally result due to a temperature change are locally restricted or even suppressed. This may be performed by a suitable selection of materials and measures which ensure that the volume of the resonator is kept constant by compensating for an enlargement of the cross-section through a reduction of the length. Further similar examples may be inferred from U.S. Pat. No. 4,706,053 and U.S. Pat. No. 6,529,104, which also each suggest means for keeping the volume of a resonator nearly constant in the event of a temperature change.
Other resonators are in turn manufactured from Invar® or similar materials, which have a low temperature expansion coefficient. However, Invar is expensive and difficult to process.